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Weather is an all-encompassing term used to describe all of the many and varied phenomena that occur in the atmosphere of a planet at a given time. The term usually refers to the activity of these phenomena over short periods of hours or days, as opposed to the term climate, which refers to the average atmospheric conditions over longer periods of time. When used without qualification, "weather" is understood to be the weather of Earth.

Weather most often results from temperature differences from one place to another, caused by the Sun heating areas near the equator more than the poles, or by different areas of the Earth absorbing varying amounts of heat, due to differences in albedo, moisture, and cloud cover. Surface temperature differences in turn cause pressure differences. A hot surface heats the air above it and the air expands, lowering the air pressure. The resulting pressure gradient accelerates the air from high to low pressure, creating wind, and Earth's rotation causes curvature of the flow via the Coriolis effect. These simple systems can interact, producing more complex systems, and thus other weather phenomena.

The strong temperature contrast between polar and tropical air gives rise to the jet stream. Most weather phenomena in the mid-latitudes are caused by instabilities of the jet stream flow (see baroclinity) or by weather fronts. Weather systems in the tropics are caused by different processes, such as monsoons or organized thunderstorm systems.

Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. In June the Northern Hemisphere is tilted towards the sun, while in December it is tilted away, causing yearly changes in the weather known as seasons. In the mid-latitudes, winter weather often includes snow and sleet, while in both the mid-latitudes and most of the tropics, tropical cyclones form in the summer and autumn. Almost all weather phenomena can occur year-round on different parts of the planet, including snow, rain, lightning, and, more rarely, hail and tornadoes.

Related portals: Earth sciences (Atmosphere  · Atmospheric Sciences)  · Tropical cyclones Featured article  · Disasters  · Water

Selected picture

Lightning striking the Eiffel Tower - NOAA.jpg

Lightning striking the Eiffel Tower on the night of June 3, 1902. This is one of the earliest photographs of lightning in an urban setting.

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The 2008 Super Tuesday tornado outbreak was a deadly tornado outbreak which affected the Southern United States and the lower Ohio Valley on February 5–6, 2008. In total, 87 tornadoes were confirmed in the outbreak's 15 hour span. Several destructive tornadoes struck heavily populated areas, most notably in the Memphis metropolitan area, in Jackson, Tennessee, and the northeastern end of the Nashville metropolitan area. Fifty-seven people were killed in the outbreak by tornadoes across four states and 18 counties, with hundreds injured and property damage totaling more than $500 million (USD).

The outbreak was the deadliest in the U.S. since the May 31, 1985 outbreak that killed 76 across Ohio and Pennsylvania (and also killed 12 in Ontario, Canada). It also was the deadliest tornado outbreak in both Tennessee and Kentucky since the 1974 Super Outbreak. In addition to the tornadoes, the same system produced significant straight-line wind damage, hail as large as softballs, or 4.50 inches (11 cm) in diameter, major flooding, significant freezing rain, and heavy snow across many areas of eastern North America.

Map of deadly tornadoes (in red) and other severe weather reports.

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Did you know...

...that Hurricane Debbie is the only known tropical cyclone ever to strike Ireland?

...that the Tempest Prognosticator, one of the earliest attempts at a weather prediction device, employed live leeches in its operation?

...that eyewall replacement cycles are among the biggest challenges in forecasting tropical cyclone intensity?

...that the Braer Storm of January 1993 is likely the strongest extratropical cyclone ever recorded in the north Atlantic Ocean?

...that in medieval lore, Tempestarii are magicians with the power to control the weather?

...that the omega equation is essential to numerical weather prediction?

Recent and ongoing weather

This week in weather history...

June 17

1946: One of the deadliest tornadoes in Canadian history struck Windsor, Ontario.

June 18

1982: A subtropical storm struck eastern Florida, causing flooding and tornadoes which killed three people.

June 19

1919: A violent tornado killed 57 people in Fergus Falls, Minnesota.

1959: The Escuminac Hurricane sunk 22 fishing boats off New Brunswick, killing 35 people.

June 20

1957: The Fargo Tornado, an extremely damaging F5 tornado, killed 11 in Fargo, North Dakota.

June 21

2007: Cyclone Yemyin developed as a tropical depression east of India, eventually crossing the Deccan Plateau to the Arabian Sea and making a second landfall in Pakistan. Almost 1000 people were killed.

June 22

1919: A tornado killed 57 people in Fergus Falls, Minnesota.

1972: Hurricane Agnes made landfall as a strong tropical storm near New York City. It would cause devastating flooding inland, killing 113 people and causing more than $2 billion in damage (1972 USD).

2007: The first F5 tornado in Canadian history struck the town of Elie, Manitoba.

June 23

1544: Artemius of Verkola, a saint in the Russian Orthodox Church, was killed by lightning.

1944: An unusual long-lived tornado tracked through mountainous West Virginia, killing 100 people.

Selected biography

Anders Celsius. Portrait by Olof Arenius (1701-1766)

Anders Celsius (November 27, 1701 – April 25, 1744) was a Swedish astronomer who is best known for his pursuit to develop a standardized temperature scale. He determined that the melting point and boiling point of water are constant regardless of latitude, and the boiling point of water is dependant on elevation.

In 1742 he proposed a temperature scale which now bears his name, the Celsius scale, which is used worldwide for meteorological observations around the world. In his scale, the boiling point of water was 0 degrees and the freezing point was 100, while the modern Celsius scale is the reverse of this. In addition to his temperature work, he was an avid observer of the aurora borealis and participated in an expedition to measure an arc of the meridian in northern Sweden.

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WikiProject Meteorology is a collaborative effort by dozens of Wikipedians to improve the quality of meteorology- and weather-related articles. If you would like to help, visit the project talk page, and see what needs doing.

WikiProject Severe weather is a similar project specific to articles about severe weather. Their talk page is located here.

WikiProject Tropical cyclones is a daughter project of WikiProject meteorology. The dozens of semi-active members and several full-time members focus on improving Wikipdia's coverage of tropical cyclones.

WikiProject Non-tropical storms is a collaborative project to improve articles related to winter storms, wind storms, and extratropical weather.

Wikipedia is a fully collaborative effort by volunteers. So if you see something you think you can improve, be bold and get to editing! We appreciate any help you can provide!

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